Formation of Graphene Oxide Nanocomposites from Carbon Dioxide

Jan 17, 2012 - We have investigated the reaction of CO2 with ammonia borane (AB), both molecules being able to function as either an acid or a base, t...
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Formation of Graphene Oxide Nanocomposites from Carbon Dioxide Using Ammonia Borane Junshe Zhang,† Yu Zhao,‡ Xudong Guan,§ Ruth E. Stark,§ Daniel L. Akins,‡ and Jae W. Lee*,† †

Department of Chemical Engineering Department, ‡Chemistry Department and Center for Analysis of Structures and Interfaces, and §Chemistry Department and CUNY Institute for Macromolecular Assemblies, The City College of New York, New York 10031, United States

bS Supporting Information ABSTRACT: To efficiently recycle CO2 to economically viable products such as liquid fuels and carbon nanomaterials, the reactivity of CO2 is required to be fully understood. We have investigated the reaction of CO2 with ammonia borane (AB), both molecules being able to function as either an acid or a base, to obtain more insight into the amphoteric activity of CO2. In the present work, we demonstrate that CO2 can be converted to graphene oxide (GO) using AB at moderate conditions. The conversion consists of two consecutive steps: CO 2 fixation (CO2 pressure 99.8% was purchased from T. W. Smith. Hydrochloric acid (37 wt % in water) was supplied by SigmaAldrich. Methanol with a purity of 99.9% was acquired from Fisher Scientific. All chemicals were used as received without further purification. Deionized (DI) water was produced in our lab with a resistivity of 18 MΩ cm 1. 2.2. Reaction of CO2 with NH3BH3. The reaction was performed using a high-pressure differential scanning calorimeter (Micro-DSC VII, SETARAM). A detailed description of this equipment has been provided in our prior work.20 The total volume of the sample cell and pipelines with fittings was 5.26 cm3. Exteriors of the pipelines and fittings were mostly exposed to the normal ambient condition, while only the sample and reference cells were subjected to the heating and cooling cycles. Gauge pressure was monitored using an ITS-5 M pressure transducer (ONEhalf20, 0 34.47 MPa) with an uncertainty in the pressure measurement of 0.05 MPa. The temperature, heat flow, and pressure of the sample cell were acquired by the SETSOFT interface. After 8.8 mg of NH3BH3 was loaded into the sample cell (made of stainless steel) at normal ambient conditions, the cell was pressurized with CO2 to a preset value. The sample and reference cells were kept at 25 °C for 5 min, and then heated to 65 °C at 1.00 °C min 1, followed by heating to 100 °C at 0.25 °C min 1. Both were then cooled to 25 °C at 3.00 °C min 1 and subsequently kept at this temperature for 5 min. Once the heating/cooling cycle was completed, the sample cell was depressurized to atmospheric pressure, and then opened to the normal ambient condition. The solid product was collected and stored in a glass vial at normal temperature and pressure. 2.3. Thermolysis of NH3BH3. About 5.0 mg of NH3BH3 was loaded into the sample cell of the Micro-DSC VII under normal ambient conditions. The sample and reference cells were kept at 25 °C for 5 min, and then heated to 115 °C at 0.50 °C min 1; both were then cooled to 25 °C at 3.00 °C min 1, and maintained at this temperature for 5 min. Once the heating/cooling cycle was completed, the sample cell was depressurized to atmospheric pressure, and then opened to normal ambient conditions. The thermolysis product, referred to as polyaminoborane, was collected and stored in a glass vial at normal ambient conditions. 2.4. Pyrolysis of Solid Product. The pyrolysis was carried out on a combined thermogravity-differential scanning calorimeter (Q600, TA Instruments) under 0.1 MPa N2 (gauge pressure) with at a flow rate of 100 cm3 STP min 1. After the solid product of CO2 reaction with NH3BH3 (between 6 and 10 mg) was loaded to the sample cup (made of alumina), the sample and reference cups were heated to 50 °C at 10.0 °C min 1, then held at this temperature for 5 min, followed by heating to the target temperature at 5.0 °C min 1. Finally, the cups were radiantly cooled to room temperature. The solid residue was collected and stored in a glass vial under ambient temperature and pressure. 2.5. Treatment of Solid Residue. All treatment was performed at normal ambient conditions. About 5 mg of solid residue, generated from the pyrolysis of solid product (produced at a CO2 pressure of 2.96 MPa) at temperatures up to 750 °C in nitrogen, was put into a 10 cm3 glass vial, following by addition of 10 cm3 of 5 M HCl, resulting in a brownish suspension of fine particles. The suspension was placed in a closed vial for 3 days, following which black particulates precipitated, creating a clear

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supernatant. The liquid was withdrawn with a 10 cm3 Transferpette (BRAND) pipet, to which 10 cm3 of DI water was added, followed by capping the vial. Again, a brownish suspension formed, which after 1 day precipitated, again resulting in a clear supernatant. The procedure of withdrawing the clear liquid and adding 10 cm3 of DI water was repeated two more times, and finally 10 cm3 of methanol was used for washing. After the clear supernatant was withdrawn in the last washing step, the resultant black particles were kept in the open vial for several hours and subsequently dried overnight in an oven at ca. 120 °C. 2.6. Characterization. ATR-FTIR (attenuated total reflection Fourier-transform infrared) spectra were recorded on a Varian 7000 FTIR using MIRacle ATR accessory, the internal reflection element of which is a Ge single reflection plate. FTIR spectra were recorded at a resolution of 2 cm 1 from 850 to 4000 cm 1 using the Varian Resolution Pro. 4.01. Raman spectra were obtained by using an HR800 Horiba Jobin Yvon micro-Raman system equipped with a CCD detector and an Olympus BX41 microscope with a 100 objective lens. Samples were placed on a cover glass and excited with 632.8 nm HeNe laser radiation. The spectrum was obtained by multiple- spectral bandpasses between 500 and 3500 cm 1 using the Labspec 4.18 software. 11B and 13C solid-state MAS NMR (magic angle spinning-nuclear magnetic resonance) spectra were recorded at 150.74 and 192.34 MHz, respectively, on a Varian VNMRS system equipped with BioMAS probe (3.2 mm), operating in a 14.1 T magnet. Samples were spun at 10 kHz for 11B and 9 kHz for 13C with the sample temperature maintained at 25 °C (ref 21). The 13C solid-state MAS NMR spectra were referenced to the methylene carbon of adamantane at 38.48 ppm (ref 22). The chemical shifts of 11B were referenced to 0 ppm based on a calculation using frequency ratios of 11B and 13C, as recommended by IUPAC guidelines.23 All data were processed with 100 Hz line broadening. TEM (transmission electron microscopy) images were recorded on a Zeiss EM 902 transmission electron microscope at an accelerating voltage of 80 kV using the Mega View III-iTEM digital image acquisition system. The line and point resolutions of TEM are 0.34 and 0.5 nm, respectively. AFM (atomic force microscope) images were acquired using Thermomicrope Explorer head equipped with a Vecco 1950-00 ultrasharp tip, and using the contact mode with SPMLab NT 5.01. Optical microscope images were recorded using the above micro-Raman system. Samples for TEM observations were prepared by dispersing about 1 mg of treated solid residue in 20 cm3 of DI water. They were subject to ultrasonication for 15 min to form a suspension of fine particles. A drop of the resultant suspension was placed onto a copper grid coated with a layer of amorphous carbon; the grid was then dried at normal ambient conditions. Samples for AFM and microscopy measurements were prepared by fast dipping a silicon wafer coated with a 100 nm SiO2 layer into the above suspension followed by drying the wafer at normal ambient conditions after withdrawing it from the suspension.

3. RESULTS AND DISCUSSION In calorimetric investigations of CO2 reaction with NH3BH3, a single resolved exothermic peak is observed as the temperature is varied from 25 to 100 °C (Figure 1). The exothermic peak is associated with the conversion of CO2 to a solid compound and the thermal decomposition of NH3BH3; the latter process is known to be promoted by CO2.24 Figure 1 shows enthalpograms obtained for the reaction at CO2 pressures of 0.845, 1.53, 2.27, 2640

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Figure 1. Enthalpograms for CO2 reaction with NH3BH3 at CO2 pressures of 0.845, 1.53, 2.27, and 2.96 MPa. The pressure specified pertains to the start of heating, and the heating rate changed from 1.00 to 0.25 °C min 1 at 65 °C.

and 2.96 MPa. The CO2 pressure is quoted as the value at 25 °C (Supporting Information, Figure S2). For the first three pressures, the occurrence of carbon fixation is inferred from the enthalpy of the exothermic event (ΔH = 63 to 95 kJ mol 1), which is significantly greater in magnitude than the enthalpy of NH3BH3 thermolysis (ΔH = 22 kJ mol 1).25 As Figure 1 shows, the peak temperature (where the heat flow reaches a maximum) decreases monotonically from 83.4 to 76.6 °C as the pressure increases from 0.845 to 2.27 MPa. For the highest pressure (2.96 MPa), the heat flow profile displays a major exothermic event at 74.6 °C and a minor event at 82.6 °C, and we find that the mass of solid product is twice that of the NH3BH3 initially present (the final mass of the solid product after the CO2 fixation is 17.2 ( 0.5 mg from 16 repeated experiments). On the basis of the preceding observations, the first exothermic peak is attributed to carbon fixation accompanying NH3BH3 thermolysis; however, the second minor event needs additional study before its origin can be determined. To obtain more insight into the proposed carbon fixation process, we characterized the solid product of CO2 reaction with NH3BH3 using micro-Raman spectroscopy. As shown in Figure 2, for CO2 pressures of 0.845 and 1.53 MPa, the spectra display structural features reported previously for the solid product resulted from CO2 reduction and NH3BH3 thermolysis.24,26 Bands centered at ca. 1461, 2957, and 3423 cm 1 are assigned to CH3 rocking, C H, and sp2 N H stretching modes, respectively. For CO2 pressures of 2.27 and 2.96 MPa, the bands assigned to sp3 N H and sp3 B H stretching modes for the solid product of NH3BH3 thermolysis are weak (Figure 2). By contrast, bands attributable to B O C bending, C O, B O, and CdO stretching modes, become stronger as the CO2 pressure is increased. On the basis of enthalpies of exothermic events and Raman spectroscopic data for the solid product, we conclude that carbon fixation and NH3BH3 thermolysis both occur with the former process becoming favorable at high CO2 pressures. The assignment of Raman bands associated with the solid product of carbon fixation is further supported by the 11B and 13C solid-state MAS NMR (magic-angle spinning nuclear magnetic resonance) investigations on the sample produced at a CO2

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Figure 2. Raman spectra for NH3NH3, polyaminoborane, and the solid product of CO2 reaction with NH3BH3 at CO2 pressures of 0.845, 1.53, 2.27, and 2.96 MPa.

pressure of 2.96 MPa. In the 11B MAS NMR spectrum (Figure 3a), a sharp peak at 1.5 ppm and a broad peak at 16.2 ppm are observed, corresponding to tetrahedral BO4 groups (sp3 B) and trigonal BO3 groups (sp2 B), respectively.27 On the other hand, the 13C MAS NMR spectrum displays three principal peaks at 25.2, 50.9, and 165.7 ppm besides the CO2 resonances (Figure 3b),24 which are assigned to aliphatic, methoxy (OCH3), and formate (HCOO) groups, respectively.28 The formation of OCH3 and HCOO groups has been reported in the solid product of CO2 reaction with NaBH4 or LiBH4,29,30 but the aliphatic group was not found in the solid product of these two reactions. Thus, by combining spectroscopic investigations and previous studies on the CO2 reduction by metal borohydrides, we deduce that the solid product of CO2 reaction with NH3BH3 contains BOCH3 and BOOCH groups. Pyrolysis of the solid product that results from CO2 reaction with NH3BH3, decomposing it at temperatures up to 750 °C in nitrogen (Supporting Information, Figures S4 and S5), yields a yellowish-brown solid residue for the samples produced at CO2 pressures of 0.845 and 1.53 MPa, and a black solid residue for the samples produced at CO2 pressures of 2.27 and 2.96 MPa (Supporting Information, Figure S7). For the latter two cases, the yields of solid residue at temperatures up to 750 °C are about 65% and 60%, respectively (Supporting Information, Figure S4); moreover, we found that 600 °C is the lowest temperature for generation of the black solid residue from the solid product resulted CO2 reaction with NH3BH3 at a CO2 pressure of 2.96 MPa (Supporting Information, Figure S8). Attempts to characterize the yellowish-brown solid residue using the microRaman were precluded by high fluorescence intensity; in continuing studies, we plan to use a low temperature coldfinger to minimize the emission. For the black solid residue, however, the Raman spectra are consistent with that for the graphitic carbon (Figure 4a and Figure S9 in the Supporting Information). Specifically, over the spectral range of 500 3500 cm 1, two sharp bands centered at ca. 1350 and 1590 cm 1 are observed; they are attributable to breathing (D band) and stretching (G band) modes of sp2 carbon carbon bonds, respectively.31,32 To further understand the pyrolysis of the solid product (produced at a CO2 pressure of 2.96 MPa), we acquired the 11B and 13C solid-state MAS NMR of the solid residue. 2641

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Figure 3. 11B and 13C MAS NMR spectra for the solid product of CO2 reaction with NH3BH3 and its correspondent solid residue. The solid product was produced at a CO2 pressure of 2.96 MPa, and pyrolysis of the solid product (at temperatures up to 750 °C) in nitrogen generated a solid residue. (a) 11 B MAS NMR. (b) 13C MAS NMR. “*” indicates spinning side bands.

Figure 4. Raman and ATR-FTIR spectra for the pristine and treated solid residue. (a) Raman spectra for the pristine and treated solid residue. (b) FTIR spectra for the pristine and treated solid residue.

The 11B MAS NMR spectrum exhibits a strong, broad peak at 12.1 ppm and a downfield shoulder (Figure 3a). The peak is attributed to trigonal BO3 groups (sp2 B). In addition to the broad peak, another small feature is observed at 1.5 ppm, which is associated with tetrahedral BO4 groups (sp3 B); no other B-containing species are detected. In the 13C solid MAS NMR spectrum, only peaks related to CdC and CdO bonds (sp2 C) are observed (Figure 3b). MAS NMR structural characterizations coupled with micro-Raman investigations lead us to hypothesize that the formation of graphitic carbon requires both formate and aliphatic groups, with methoxy groups converting to gaseous species, because the pyrolysis of sodium formatomethoxyborane

(NaBO(OCH3)(OOCH)) produces methyl borate, methyl formate, boron oxide, and sodium salts.30 As noted above, the solid residue from pyrolysis of the solid product (produced at a CO2 pressure of 2.96 MPa) contains graphitic carbon. To identify the type of carbon, we treated the resultant residue sequentially with 5 M HCl, deionized water, and methanol and then characterized the treated residue using micro-Raman, ATR-FTIR (attenuated total reflection Fourier-transform infrared spectroscopy), optical microscopy, TEM (transmission electron microscopy), and AFM (atomic force microscopy). The treatment has little effect on the state of the graphitic carbon (Figure 4a), but appears to dehydrate the 2642

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Figure 5. Characterization of the treated solid residue. (a) Microscopic images indicating there are flakes in the solid residue. (b and c) Representative TEM micrographs showing curled and stacked flakes. (d) A representative AFM image demonstrating that the thickness of one flake is less than 6 nm.

boron-containing compounds, because both the in-plane B O H bending and O H stretching modes diminish in intensity (Figure 4b). After the treatment, the Raman spectrum for the solid residue displays a weak broad band centered at ca. 2685 cm 1 (Figure 4a); it is assigned to the 2D band.33,34 Optical microscopy images of treated residue reveal the presence of transparent flakes (Figure 5a). Also, the presence of flakes is further supported by the TEM images (Figure 5b,c). The thickness of a typical flake is less than 6 nm, as determined from the AFM images (Figure 5d). On the basis of microscopy studies, NMR, and Raman spectroscopic observations, we deduce that graphene oxide is produced during pyrolysis. Furthermore, we view the treated solid residue as a graphene oxide boron oxide sandwich-like nanocomposite because boron oxide cannot be totally removed by acid- and water-washing (Figure 4b).

4. CONCLUSION In summary, from a conflation of microscopy, magnetic resonance, and Raman spectroscopy measurements, we have demonstrated that CO2 can be converted to graphene oxide nanocomposites by NH3BH3 via two consecutive steps: NH3BH3 first reduces CO2 to form a solid compound at temperatures less than 100 °C and CO2 pressures below 3 MPa (carbon fixation), and,

subsequently, the solid product decomposes in an inert atmosphere at temperatures above 600 °C to generate graphene oxide (graphenization). Moreover, our study reveals that graphene oxide obtained from CO2 reacting with NH3BH3 may have properties different from those prepared from solid carbon sources like graphite.13 These findings present a new approach for converting CO2 from industrial and natural sources to a variety of graphenic materials. Moreover, the formation of C H and C C bonds through CO2 activation is of great interest in synthesizing liquid fuels and useful chemical from CO2. To fully explore the method developed here, our ongoing work includes detailed mechanistic studies of both carbon fixation and graphenization, coupled with expanded structure determinations for the product of carbon fixation and the proposed graphene oxide nanocomposites. Also, efforts are made to produce other organic compounds from the solid product of CO2 reaction with NH3BH3.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information of CO2 reaction with AB, characterization of the solid product, thermal decomposition of the solid product, and characterization of the 2643

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: (212) 650-6688. Fax: (212) 650-6660. E-mail: lee@che. ccny.cuny.edu.

’ ACKNOWLEDGMENT We are grateful for the financial support of the NSF and NIH under grant numbers HRD-0833180, MCB-0741914, and 2G12 RR03060. ’ REFERENCES

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